Since a transmissive scattered liquid crystal (also termed a polymer scattered liquid crystal) does not include a deflection plate, the transmittance thereof is high when it displays data. Thus, the transmissive scattered liquid crystal is very suitable for an application which requires a high transmittance. However, when a DC voltage is applied to a transmissive scattered liquid crystal, an electrolysis takes place, thereby reducing the life of the liquid crystal. Therefore, when the liquid crystal is driven, an AC voltage should be applied.
Known drive methods for transmissive scattered liquid crystal display devices include a static drive method and a multiplex drive method. In the static drive method, one end of an electrode of each pixel is connected to one common line so as to switch the other end thereof. In the multiplex drive method, which uses two or more common electrodes, each common electrode is selected and driven on a time division basis. FIG. 1 shows a conceptional schematic diagram of the static drive method. FIG. 2 shows a conceptional schematic diagram of the multiplex drive method.
FIG. 3 shows a circuit diagram of the static drive method for one pixel 1 of a liquid crystal display device. FIG. 4 shows a drive waveform diagram for the circuit of FIG. 3. An exclusive OR gate 2 is used as the drive circuit for the pixel 1 of the liquid crystal display device. When a square shaped AC voltage V.sub.c is applied to one electrode C of the pixel 1 of the liquid crystal display device and to one input of the exclusive OR gate 2 while an ON portion of the ON/OFF signal V.sub.sig is being applied to another input of the exclusive OR gate 2, an inverted voltage V.sub.8, wherein the plus and minus of the square shaped voltage V.sub.c is inverted by the exclusive OR gate 2, is applied to the other electrode S. Thus, a liquid crystal applying voltage (V.sub.c -V.sub.s) is applied between the electrodes C and S of the pixel 1, and thereby data appears on the pixel. In contrast, when the ON/OFF signal V.sub.sig is turned off, the voltage applied to the pixel 1 of the liquid crystal display device becomes zero, and thereby the data on pixel 1 disappears. While a single common electrode can be connected to each pixel, this circuit configuration requires a number of exclusive OR gates 2 corresponding to the number of pixels in order to be able to apply a control signal to each pixel of the liquid crystal display device.
Although the multiplex drive method requires a plurality of common electrodes, it can drive the liquid crystal display device with a lesser number of data electrodes (i.e., number of all pixels/number of common electrodes) than that of the static drive method. Thus, the multiplex drive method is suitable for driving a liquid crystal display device with a large area and a large number of pixels. However, when the liquid crystal display device is driven in the multiplex drive method, an AC voltage should be applied as in the static drive method in order to avoid reducing the life of the liquid crystal display device. Thus, the plus and minus of a drive voltage for the liquid crystal according to the multiplex drive method should be changed as shown in FIG. 5.
FIG. 6 shows a method for applying voltages to pixels of a liquid crystal with first and second common electrodes C.sub.1 and C.sub.2 and first, second, and third data electrodes D.sub.1, D.sub.2, and D.sub.3. FIG. 7 shows waveforms of voltages applied to these pixels. In FIG. 6, pixels G.sub.1, G.sub.2, and G.sub.4 denoted by solid black dots are non-display or unlit pixels, whereas pixels G.sub.3, G.sub.5, and G.sub.6 denoted by white dots are display or lit pixels.
The drive voltages of the transmissive scattered liquid crystal display devices according to the static drive method and the multiplex drive method are high. For example, these liquid crystal display devices normally require a drive voltage as high as 40 V as effective values. Thus, the drive voltage according to the static drive method is higher than the withstand voltage of a dedicated integrated circuit (IC) which performs a static drive. Therefore, discrete devices such as power transistors and Triac devices have been used to provide a number of drive circuits corresponding to the number of pixels. For example, when a liquid crystal display panel of 24 dots by 24 dots is driven according to the static drive method, 576 drive circuits should be formed in order to separately drive the 576 pixels.
On the other hand, a drive voltage of around 40 V is required for operations according to the multiplex drive method. This drive voltage is higher than the withstand voltage of a dedicated IC which drives a conventional TN (Twisted Nematic) type liquid crystal display device according to the multiplex drive method. Thus, as in the static drive method, a number of circuits corresponding to the number of pixels have been formed with discrete devices such as power transistors and Triac devices. For example, to drive a liquid crystal display panel having 72 dots by 36 dots according to the multiplex drive method by using two common electrodes, 1296 drive circuits corresponding to 2592 pixels (because 72.times.36/2=1296) should be formed.
In both the static drive method and the multiplex drive method, a large number of parts are required when the drive circuits are formed of discrete devices. Accordingly, the following problems result:
A very large space is required. PA1 The reliability of the drive circuits is degraded. In particular, power devices adversely affect the drive circuits. PA1 When a defect occurs, troubleshooting and/or maintenance takes a long time.
If the drive circuits were to be formed as part of an IC, the number of parts could be remarkably reduced. However, since the drive voltage of the transmissive scattered liquid crystal display device is high, it cannot be driven by a dedicated IC.
Japanese Published Unexamined Patent Application (A) 4-325284 and Japanese Published Unexamined Patent Application (A) 5-42379, assigned to the assignee of the present application, disclose a technique for using a transmissive scattered liquid crystal display device as a mask of a laser marker. In addition, the inventors of the present invention are developing a technique for driving such a liquid crystal according to the multiplex drive method.
Before data is written on a liquid crystal display device, the liquid crystal display device should be satisfactorily discharged. Otherwise, residual portions of a previously produced image may remain on the display, thus degrading the marking quality. Therefore, in developing a technique for driving a liquid crystal display device according to the multiplex drive method, countermeasures for preventing a residual image should be taken.
An example of such a residual image protection technique is disclosed in Japanese Published Unexamined Patent Application (A) 1-134497. According to this technique, when an image on the liquid crystal display device is to be changed, a voltage in a non-display level is applied to an electrode of each pixel just before a voltage supply route to both electrodes of each pixel is shut off.
In recent years, the need for high speed printing with a laser marker has become strong. The inventors of the present invention applied the technique disclosed in Japanese Published Unexamined Patent Application (A) 1-134497 to the laser marker that they are developing. As an experimental result, when the printing speed was low, an effect was confirmed. However, when the printing was performed at a high speed, a residual image remained. Thus, the marking quality was degraded.